Devices for estimating the stress in concrete structures

ABSTRACT

A reinforced-concrete structure is provided with a bore extending into the concrete mass from one face and an annular member, whose deflection in terms of applied stress is known, set in the wall of the bore, the member having a linear dimension measurable from within the bore which is indicative of the deflection.

United States Patent Inventor Alfred Norman Kinkead Winlrith, England Apple No. 888,306 Filed Dec. 29, 1969 Patented Sept. 7, 1971 Assignee United Kingdom Atomic Energy Authority London, England Priority Jan. 10, 1969 Great Britain 1731/69 DEVICES FOR ESTIMATING THE STRESS 1N CONCRETE STRUCTURES 1 Claim, 2 Drawing Figs.

' U.S. Cl 73/88 E Int. Cl G01b 5/30 Field of Search 73/88 R, 88

E, 88.5 R, 141 A;33/1 H, 147 1) [56} References Cited UNlTED STATES PATENTS 1,670,981 5/1928 Perkins et a1. 73/88 EX 2,898,761 8/1959 Hast 73/88 E 3,132,319 5/1964 Brooks, Jr. 73/885 R X 3,273,382 9/1966 Fonash 73/885 R 3,447,367 6/1969 Taylor 73/885 R FORElGN PATENTS 226,589 6/1959 Australia 73/88 E 151,797 10/1955 Sweden 73/88 E Primary Examiner-Char1es A. Ruehl AttorneyLarson, Taylor & Hinds ABSTRACT: A reinforced-concrete structure is provided with a bore extending into the concrete mass from one face and an annular member, whose deflection in terms of applied stress is known, set in the wall of the bore, the member having a linear dimension measurable from within the bore which is indicative ofthe deflection DEVICES FOR ESTIMATING THE STRESS IN CONCRETE STRUCTURES BACKGROUND OF THE INVENTION This invention relates to concrete structures which operate under stress. The invention chiefly relates to concrete pressure vessels which have been prestressed to eliminate, as far as is practicable, the formation of tensile stresses. The specification will describe the invention in terms of its application to prestressed-concrete pressure vessels.

Prestressed-concrete pressure vessels are usually instrumented heavily, even when the particular design in not a novel one, with the object of obtaining information about stresses which develop in the wall under various mechanical and thermal loads and of monitoring the effectiveness of the prestressing system. As a result of this need to monitor stresses in massive concrete it has been common practice, hitherto, to use embedded strain gauges at various parts of the concrete from which, one can calculate the stress, knowing the modulus. The validity of this method depends upon the modulus remaining substantially constant. However it can be shown that this value varies widely during the life of the concrete. It is evident, therefore, that advantage is to be gained if the stress in the concrete be measured directly and if this measurement can be monitored while the pressure vessel is in use.

SUMMARY OF THE INVENTION According to the invention a reinforced-concrete structure is provided with a bore extending into the concrete mass from one face thereof and an annular member set in the wall of the bore, an annular member whose deflection in terms of applied stress is known, the member having a linear dimension measurable from within the bore which is indicative of the deflection. a

DESCRIPTION OF THE DRAWING In order that the invention may be better understood a practical application thereof will now be described with reference to the accompanying drawing which shows in FIG. I a medial cross section through a bore in a massive structure which forms part of a prestressed concrete pressure vessel;

FIG. 2 a cross section on the line IIlI of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The bore 1 extends through the concrete wall 2 being closed on the inner face of the vessel by the vessel liner 3 carrying the usual heat insulation 4. At the outer face of the wall there is a collar 5 which has a number of stud fastenings for securing a closure plate not shown. The bore wall is lined throughout its length with a steel thimble 6. Attention is directed to two enlarged-diameter portions of the bore namely at axially spaced positions 7 and 8.

At each enlarged-diameter portion an annular member is set into the wall of the bore. The annular member comprises two resilient elliptical rings, 9, 10 fixed together in planes at right angles and whose deflection in terms of applied stress in known. The inner surface of each elliptical ring is shaped as a knife edge as at 9a, 10a, providing reference surfaces from which diametric measurements may be made to gain an indication of deflections of the rings and hence of the surrounding concrete. To key each ring to the concrete a number of hooks, as at 11 are welded to the periphery of the rings. The annular members may be assembled in a preformed thimble which is placed in position in preparation for concreting; as it is laid, cover plates fitted over the outer mouth of the thimble will ensure cleanliness of the interior until and after the concrete has set. When it is required to check the stress in the concrete, the cover plate is removed and a pair of precision calipers is inserted within the thimble into a position adjacent the annular member and the calipers expanded to measure the diametric distances between the knife edges. These measurements are compared with those taken in the as manufactured condition and any difference can be used to determine stresses (in planes at right angles) to which the annular members and hence the concrete has been subjected, due allowance for temperature differences being made in the calculation.

If desired, a dial gauge transducer or other measurement device may be set up within the thimble to measure any deflection of the annular members, and this may be trans mitted to a data logger, located at some central position, receiving similar information from bores at other positions in the structure. In this way the whole stress patterns in the concrete structure could be continuously monitored.

In cases where stresses are known to remain essentially normal to the axis of the thimble, single circular annular rings may be employed at each measuring point in place of the crossed elliptical rings described, with considerable simplification to the thimble unit and to the measuring device.

I claim:

l. A reinforced-concrete structure having a bore extending into the concrete mass from one face thereof and a member set into the bore wall comprising two rings lying in intersecting planes at right angles to one another, the rings being joined together where they intersect, the deflection of the member in terms of applied stress being known such that deflection measurements can provide a measure of stress within the concrete. 

1. A reinforced-concrete structure having a bore extending into the concrete mass from one face thereof and a member set into the bore wall comprising two rings lying in intersecting planes at right angles to one another, the rings being joined together where they intersect, the deflection of the member in terms of applied stress being known such that deflection measurements can provide a measure of stress within the concrete. 